1. Field of the Invention
The present invention relates to a centrifugal compressor used in, for example, a small gas turbine or turbo refrigerating machine, and especially relates to a centrifugal compressor equipped with a typical vane type diffuser or a diffuser what is called a channel type diffuser.
2. Description of the Related Art
Centrifugal compressors are provided with a diffuser that functions as an apparatus that reduces the velocity of a gas and converts its kinetic energy into internal energy. An example of a centrifugal compressor provided with a diffuser is shown in FIGS. 9 and 10. The centrifugal compressor shown in the drawings is equipped with a casing 1, an impeller 2 that rotates by being axially supported by the casing 1, a scroll 3 provided integrated into a single unit with the casing 1 around the impeller 2, and a diffuser 4 provided in the shape of a ring so as to surround the impeller 2 between the impeller 2 and scroll 3.
The diffuser 4 is composed of a plurality of vanes 5 disposed separated from each other in the peripheral direction, and fulfills the function of moving the direction of flow of gas discharged from the impeller 2 closer to the outside in the radial direction, while also reducing the velocity to convert the dynamic pressure of the gas into static pressure.
However, in a centrifugal compressor as described above, since the inflow angle of air to the diffuser 4 changes when the intake flow volume of the impeller 2 is changed, even if for example, the direction of flow of gas discharged from the impeller 2 coincides with the direction of a wing center line on the front edge of the vane 5 at a certain intake flow volume, if the intake flow volume changes, both no longer coincide resulting in a decrease in diffuser efficiency. This causes the operating range from surge to choke to become narrower.
Therefore, although attempts have been made to reduce the ratio of chord length to the pitch between the vanes (chord-pitch ratio) and prevent the formation of a throat portion between the adjacent vanes in order to widen the operating range, this makes it difficult for conversion to static pressure to proceed and resulting in the problem of being unable to obtain adequate diffuser efficiency. Here, the throat portion refers to a space between the adjacent vanes extending from a line dropped down vertically from the front edge of the one vane to the wing center line to a line dropped down vertically from the rear edge of the other vane to the vane center line.
In consideration of the circumstances as described above, the object of the present invention is to provide a centrifugal compressor that allows a wider operating range from surge to choke by inhibiting decreases in diffuser efficiency even if the intake flow volume of the impeller is changed.
As a means for solving the above problems, a centrifugal compressor is employed having the structure described below. Namely, the first aspect of the present invention is a centrifugal compressor having a diffuser around an impeller; wherein, the diffuser is equipped with a plurality of vane groups comprised of a plurality of vanes disposed in the peripheral direction of the impeller so as to be concentric about the center of an axis of rotation of the impeller, and the more the vane belongs to the vane group positioned to the outside, the smaller the angle relative to the radial direction of the impeller.
In this centrifugal compressor, since conversion from dynamic pressure to static pressure for gas exhausted from the impeller proceeds with each passage of the gas through each vane group disposed in concentric circles, high efficiency is obtained when the gas passes through the outermost positioned vane group.
The second aspect of the present invention, is the centrifugal compressor according to the first aspect wherein, in any vane group excluding the vane group at a position nearest the impeller, the number of vanes belonging to the vane group is an integral multiple of the number of vanes belonging to the other vane group adjacent on the inside to the vane group.
The flow of gas discharged from the impeller is organized along vanes belonging to the vane group positioned nearest to the impeller during the course of passing through the vane group, and flow is formed such that it is curved in the direction of a wing center line behind (outside) the vanes. Conversion from dynamic pressure to static pressure proceeds with good efficiency if this flow is fed outward without weakening in each of the vane groups of the latter stage. In this centrifugal compressor, vanes that continue to send the flow of gas outward are always provided in each vane group, except for the vane group positioned nearest to the impeller, corresponding to individual vanes belonging the vane group positioned nearest to the impeller. As a result, conversion from dynamic pressure to static pressure proceeds with good efficiency thereby allowing the obtaining of high diffuser efficiency.
The third aspect of the present invention, is the centrifugal compressor according to the first or second aspect wherein, at least the vanes belonging to the vane group at the position nearest to the impeller are able to rotate individually by being axially supported by shafts parallel to the axis of rotation.
If the intake flow volume of the impeller changes, the direction of flow of gas discharged from the impeller, and the direction of the wing center line on the front edge of the vanes belonging to the vane group positioned nearest to the impeller no longer coincide, thereby making it difficult for the flow to continue and ending up decreasing diffuser efficiency. Therefore, the vanes are rotated so as to change the inclination of the direction of the wing center line on the front edge and coincide with the direction of flow of gas discharged from the impeller. As a result, diffuser efficiency is maintained at a high level if the intake flow volume of the impeller changes.
The fourth aspect of the present invention, is the centrifugal compressor according to the third aspect wherein, the rotatable vanes stand on flanges independent from the walls that form a portion of the diffuser separated in the direction of the axis of rotation with the vanes interposed between, and rotate with the flanges.
If composed so that only the vanes rotate, a gap ends up forming between the walls that form a portion of the diffuser and the vanes, which causes a disturbance in the flow of gas and a decrease in diffuser efficiency. Therefore, if the vanes stand on flanges and are rotated together with those flanges, there is no longer any gap between the walls and vanes, thereby enabling diffuser efficiency to be maintained at a high level without decreasing.
The fifth aspect of the present invention, is the centrifugal compressor according to the third or fourth aspect wherein, the vane group adjacent on the outside to the rotatable vanes is able to turn in the peripheral direction while maintaining the arrangement of the individual vanes.
Since turning the vanes causes the position of not only the front edge but also the rear edge to change, correlation with the leading edges of the vanes belonging to the vane group adjacent on the outside is no longer possible in terms of continuing the flow of gas to the outside, thereby causing a decrease in diffuser efficiency. Therefore, if the vane group adjacent on the outside to the rotatable vanes is allowed to turn in the peripheral direction while maintaining the arrangement of individual vanes, it becomes possible to correlate the rear edges of the rotatable vanes with the front edges of the vanes belonging to the vane group adjacent on the outside under any circumstances, thereby enabling diffuser efficiency to be maintained at a high level without decreasing.
The sixth aspect of the present invention, is the centrifugal compressor according to the third, fourth or fifth aspect wherein, the ratio of the chord length to the interval between the adjacent vanes in the peripheral direction of the rotatable vanes is less than 1.0.
Although the angle of the rotatable vanes relative to the radial direction of the impeller is set to become larger the smaller the intake flow volume of the impeller, and smaller the larger the intake flow volume of the impeller, if the vane angle approaches 90xc2x0 by reducing the intake flow volume of the impeller (although the compressor is actually thought to become inoperable due to the occurrence of surging), interference can occur between the vanes. Therefore, if the ratio of chord length to the interval between the adjacent vanes in the peripheral direction is made to be less than 1.0, even if the vane angle becomes 90xc2x0, there is no occurrence of interference between the vanes, and operability is improved.
The seventh aspect of the present invention, is the centrifugal compressor according to one of the third to sixth aspects wherein, the ratio of the chord length to the interval between the adjacent vanes in the peripheral direction of those vanes belonging to the vane group adjacent on the outside to the rotatable vanes is from 0.5 to 2.0.
If the interval between the adjacent vanes in the peripheral direction is too open, this is not appropriate since it can cause disturbances in the flow of gas. Therefore, if the ratio of chord length to the interval between the adjacent vanes in the peripheral direction of those vanes belonging to the vane group adjacent on the outside to the rotatable vanes is made to be from 0.5 to 2.0, the gas flow is rectified thereby preventing decreases in diffuser efficiency.
The eighth aspect of the present invention, is the centrifugal compressor according to one of the first to seventh aspects wherein, the ratio of the length from the center of the impeller to the front edge of a vane belonging to the vane group at a position nearest the impeller toward the outer radius of the impeller is from 1.05 to 1.30.
Since the gas immediately after being discharged from the impeller has an uneven speed from the impeller until it enters the diffuser, the effects of the vanes are minimal, while the free vortex gaps where there are no vanes have more of an effect on improving diffuser efficiency. Therefore, if the ratio of the length from the center of the impeller to the front edge of a vane belonging to the vane group positioned nearest the impeller to the outer radius of the impeller is made to be from 1.05 to 1.30, since free vortex gaps where there are no vanes are provided to the inside of the diffuser, thereby improving diffuser efficiency.